Magnetic sensors such as those used in disk drives or tape drives often include multi-layered magnetoresistive (MR) structures. The MR structures typically include a pair of ferromagnetic layers separated by a nonmagnetic spacer layer. One of the ferromagnetic layers is designated as a free layer, meaning that the direction of magnetization is free to change in the presence of an external magnetic field. The other ferromagnetic layer is designated as a pinned layer, meaning that its magnetization direction is fixed in a particular direction.
A useful property of such an MR structure is that its electrical resistance depends on the relative orientations of the magnetization directions of the free and pinned layers. Thus, MR structures can be used for detecting the presence of an external magnetic field. For example, in disk drives a sensor having an MR structure is used to detect data stored in the form of tiny magnetic fields on a recording medium (e.g., magnetic disk).
In many applications, such as linear-response read heads for disk drives, an MR structure is needed where the magnetization direction of the free layer is perpendicular to the magnetization of the pinned layer. There are several conventional methods for providing this orthogonal orientation. One method is the so-called hard biasing method, for example as described in U.S. Pat. No. 6,756,135 to Hasegawa et al., titled “Spin Valve Thin-Film Magnetic Element.” This method has some undesirable attributes such as gradual reduction of sensitivity at track edges and is somewhat difficult to control in manufacturing.
Another common method is to set the magnetization directions of the free and pinned layers through the use of exchange coupling with adjacent antiferromagnetic (AFM) layers. The AFM layers are initialized or “set” during manufacturing to different magnetization directions that are perpendicular to each other. In order to set the magnetization direction for an AFM layer, it must be heated to a particular blocking temperature (e.g., at or near the Neel temperature of the AFM material) in the presence of a magnetic field having an appropriate field direction. Thus, the steps of heating and subsequent cooling in a field of a particular field direction would be appropriate for one of the AFM layers, but not the other. Thus, two different AFM materials are used that have distinctly different blocking temperatures. The AFM layer with the highest blocking temperature is set first. Then the field angle is rotated 90 degrees and the second AFM layer is set at a lower temperature.